I suggest you use google gears for GMail. You can setup all your other accounts within GMail so you can send as them, and with gears (watch the video), you get offline support to send/receive messages.

You could d/l the Windows Live Mail client, which is free, and seems to do what you want. But then again, so does the Windows Mail client that came with your Vista system, so there must be another consideration.

I solved a similar problem by just using my Android phone with gmail, though it is a pain to type on.